Inadequate thermalization (heating) of precursor gases prior to their injection into a CVD chamber and their premature mixing within the chamber can lead to a number of problems that can be specific to each particular CVD process being performed. Consider, as an example, the hydride-vapor-phase epitaxial (HVPE) growth of GaN using GaCl3, and NH3 as the precursor gases, where problems caused by inadequate thermalization and premature mixing include the following.
First, injection of inadequately thermalized precursors can lead to unwanted deposition on surfaces other than the growth substrate. Over time, this unwanted material can lead to increased particulate levels in the reactor sufficient to decrease wafer quality, and also to coating of chamber walls sufficient to interfere with efficient radiant heating. Such undesirable deposition occurs since GaCl3 condenses from the vapor phase at relatively low temperatures, e.g., less than 500° C., and, therefore, areas of the reactor that are not maintained above vaporization temperatures are likely to become coated. It is, therefore, desirable that GaCl3 be thermalized to temperatures of at least about 500° C. prior to injection into the reaction chamber. In fact, it is desirable to thermalize the GaCl3 precursor to temperatures of at least 730° C. prior to injection into the reaction chamber.
Further in connection with HVPE processes, injection of inadequately thermalized precursors can lead to unwanted side reactions that limit actual GaN deposition. Because gas temperatures less than about 930° C. can lead to formation of undesirable adducts, e.g., GaCl3:NH3, it is also desirable that both GaCl3 and NH3 be thermalized to a temperature of at least about 930° C. prior to injection into the reaction chamber. Moreover, to further limit formation of such undesirable adducts, it is preferable to keep separate the group III and group V precursor flows until they are in the direct vicinity of the growth susceptor. Premature mixing of the precursor gases can result in unwanted reaction by-products and the production of particulates within the reactor.
Finally, in connection with HVPE processes, it is desirable to thermalize the group V precursor (commonly NH3) prior to injection into the reaction chamber. Inadequately thermalized group V precursor, upon mixing with a thermalized group III precursor, can cool the group III to a sufficient extent to lead to the above undesirable effects. However, thermalization of the group V ammonia precursor should not be carried out in an environment containing metals, e.g., in metallic gas lines, metallic reactor components, etc., as is often done. At elevated temperatures, metals can catalyze cracking of reactive NH3 to N2 (and H2), which is not reactive with GaCl, to produce GaN.
The above problems resulting from inadequate thermalization and premature mixing result in an inefficient reaction of the precursor gases to form the GaN product at the substrate. Precursor reactants are lost due to particle/complex formation, deposition on unwanted surfaces, and so forth. Improving thermalization and delivery of precursor gases can be expected to result in a more efficient utilization of precursor gases with the associated benefits in reduced costs and improvements in material growth rate.
Problems of precursor thermalization and separation for HVPE III-nitride deposition is addressed in U.S. Pat. Nos. 6,179,913 and 6,733,591. However, this prior art is concerned with conventional equipment where GaCl is formed in situ by reacting HCl gas with liquid gallium and is not applicable to equipment that directly injects gaseous GaCl3. U.S. Provisional patent application Ser. No. 61/015,524 is concerned with thermalization and spatial separation of precursors utilizing external GaCl3 and NH3 sources, however, this prior art application utilizes a single injection fixture for injection of both the group III and group V precursors.